1. Field of Invention
This invention relates to a projection-type display apparatus enlarging and projecting color light image onto a screen through a projection lens by decomposing the white light beam from a light source into three-color light beams of red, blue, and green, modulating these colored light beams through a light valves comprising liquid crystal panels according to given video information and recombining the modulated light beams of different colors after modulation. More specifically, the present invention relates to a structure for preventing light cross talking on the dichroic prism that comprises the light synthesizer in the optical system for such a projection-type display apparatus. The invention also relates to a dichroic prism structure for accurate assembling.
2. Description of the Related Art
Fundamentally, a projection-type display apparatus comprises a light source a color separator segregating white light beam emitted from a light source into three primary color light beams, three liquid crystal light valves modulating three color light beams, a light synthesizer synthesizing modulated color light beams and a projection lens enlarging and projecting synthesized, modulated light beam onto a screen.
As color synthesizer, the dichroic prism is a well-known. For embodiment, the structure of a dichroic prism was disclosed in laid-open Japanese patent applications S39-20049 and S62-1391 by the present applicants. As described in these publications, the dichroic prism is composed of four triangle prisms of the same shape that are affixed together. On the X-shaped fixing surface, a dielectric film reflector surface is formed that has the ability selectively to reflect a given color. However, there are several notable problems in the manufacture of such prisms, as will be discussed below.
First, as indicated in FIG. 24, the modulated colored light beams that have passed through liquid crystal light valves 925 R, G, and B and that pass through prism unit 910, are reflected by its X-shaped reflecting surfaces (9100R, 9101R, 9100B, 9101B), and are emitted to the side of projection lens unit 6. A small amount of light, however, fails to be reflected by the X-shaped reflecting surface; it passes through the reflecting surface and sometimes reaches the backside of the liquid crystal light valve that is located across prism unit 910. For example, the blue modulated light beam may occasionally pass through blue reflecting surfaces 9100B and 9101B, exit from the incident surface 910R for the red light beam, and reach here from the backside of red liquid crystal light valve 925R. Conversely, the red modulated light beam may sometimes pass through red reflecting surfaces 9100R and 9101R, and reach here from the incident surface 910B for the blue light beam and from the back side of blue liquid crystal light valve 925B. Moreover, the green modulated light beam may sometimes be reflected to the side of red liquid crystal light valve 925R instead of passing through prism unit 910. Light striking the backside of liquid crystal light valve 925R in this manner could have a deleterious effect, such as causing a malfunction of the liquid crystal panel. Blue light with a short wavelength can have a particularly significant influence in this regard.
Secondly, if these four prisms are not precisely aligned in a dichroic prism that is used as a color synthesizer in a projection-type display apparatus, the images formed by the different colors synthesized through the dichroic prism thus obtained may become misaligned on the screen, thus resulting in poor image quality. For example, as shown in FIG. 20, such a problem arises when bumps exist between fixed surfaces. The following conventional method has been proposed to fix prisms accurately: for example, as shown in FIG. 21, a pair of prisms 910a and 910b from the four prisms 910a, 910b, 910c, and 910d are fixed with a step difference between them; the remaining pair of prisms, 910c and 910d, are likewise fixed with a step difference between them. After that, step difference surfaces 910e and 910f are used as positional alignment surfaces in order to affix the pairs of prisms. This method is described in Japanese application S39-20049 referenced above.
Although this method works well for positionally aligning prisms in one direction, however, it cannot align prisms in the orthogonal direction thereto. Specifically, when four prisms are fixed, the center of the X-shaped fixation surface cannot be determined accurately.